When the level editors first came out, I edited the map Lowdark to put a Cyberdemon in one of the item rooms. My buddy and I used to murder each other on that map - we'd always race each other for the good stuff in that room. He never played single player and remembering his scream when he beat me to the treasure room and found it occupied makes me smile over 20 years later.
I also played through the entire first 7 levels of the game without taking a hit for some stupid contest that was going around. It took quite a few tries.That pales compared to my buddy losing his composure though.